The invention relates to an arrangement for a system for the contactless transmission of data and/or energy between a base station, including an antenna resonant circuit, and a transponder.
The base station transmits data and energy to a transponder via an electromagnetic field. The highest transmission efficiency is achieved when the resonance frequency of the antenna resonant circuit is tuned to a transmission frequency which is defined by a generator in the base station and whereto the resonance frequency of the transponder resonant circuit is also tuned.
EP 0 625 832 A1 describes a system for the wireless transmission of data and energy in which the antenna resonant circuit is automatically tuned to a fixed transmission frequency by way of connectable capacitances.
Detuning of the antenna resonant circuit occurs due to temperature-induced drift phenomena in the elements of the resonant circuit or due to manufacture-related deviations. Consequently, the resonance frequency of the resonant circuit is no longer tuned exactly to the transmission frequency, so that the antenna current is reduced and hence the transmitted field strength also decreases. Such detuning could possibly preclude correct transmission of data and energy to the transponder, or could substantially reduce the distance that can be allowed between the base station and the transponder. In order to avoid these functional restrictions, either components having small tolerances must be used or the systems must be manually retuned. Complex circuits for automatic tuning of the transmission frequency could also be employed. Another possibility of ensuring reliable transmission is to increase the operating power of the system. However, stronger antenna drivers are then required; like the other possibilities, this increases the cost of the system.
When the resonance frequency of an antenna resonant circuit of the base station is accurately tuned to the transmission frequency, the impedance of the antenna resonant circuit consists only of the real ohmic resistance, so that the maximum current is reached and hence also the maximum field strength. The maximum current possible is limited by the antenna drivers and the power supply. Usually public rules are imposed as regards the maximum field strength. Antenna resonant circuits reach the field strength required for a data and energy transmission in a tolerance range. This tolerance range characterizes the range in which the resonance frequency may deviate from the transmission frequency without endangering reliable transmission. There is a measure for the maximum permissible deviation of the resonance frequency of the antenna resonant circuit from the transmission frequency. If the resonance frequency of the resonant circuit lies outside this tolerance range, transmission of data and energy cannot take place.
When the antenna resonant circuit is operated with a higher power, the current flowing and the maximum field strength increase if the antenna resonant circuit is tuned. The tolerance range for the resonance frequency in respect of the deviation from the transmission frequency is greater in antenna resonant circuits operating with a higher power than in antenna resonant circuits operating with less power. However, stronger drivers are required for the higher power. In addition to this additional expenditure, antenna resonant circuits operating with a higher power also generate a stronger electromagnetic field when the antenna resonant circuit is tuned, so that public rules might be violated.